[PATCH 5.12 072/127] x86/sev-es: Use __put_user()/__get_user() for data accesses

Greg Kroah-Hartman
2021-05-24T16:04:29Z

From: Joerg Roedel <jroedel@suse.de>

commit 4954f5b8ef0baf70fe978d1a99a5f70e4dd5c877 upstream.

The put_user() and get_user() functions do checks on the address which is
passed to them. They check whether the address is actually a user-space
address and whether its fine to access it. They also call might_fault()
to indicate that they could fault and possibly sleep.

All of these checks are neither wanted nor needed in the #VC exception
handler, which can be invoked from almost any context and also for MMIO
instructions from kernel space on kernel memory. All the #VC handler
wants to know is whether a fault happened when the access was tried.

This is provided by __put_user()/__get_user(), which just do the access
no matter what. Also add comments explaining why __get_user() and
__put_user() are the best choice here and why it is safe to use them
in this context. Also explain why copy_to/from_user can't be used.

In addition, also revert commit

7024f60d6552 ("x86/sev-es: Handle string port IO to kernel memory properly")

because using __get_user()/__put_user() fixes the same problem while
the above commit introduced several problems:

1) It uses access_ok() which is only allowed in task context.

2) It uses memcpy() which has no fault handling at all and is
thus unsafe to use here.

[ bp: Fix up commit ID of the reverted commit above. ]

Fixes: f980f9c31a92 ("x86/sev-es: Compile early handler code into kernel image")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org # v5.10+
Link: https://lkml.kernel.org/r/20210519135251.30093-4-joro@8bytes.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kernel/sev-es.c | 66 ++++++++++++++++++++++++++++++++---------------
1 file changed, 46 insertions(+), 20 deletions(-)

--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -288,31 +288,44 @@ static enum es_result vc_write_mem(struc
u16 d2;
u8 d1;

- /* If instruction ran in kernel mode and the I/O buffer is in kernel space */
- if (!user_mode(ctxt->regs) && !access_ok(target, size)) {
- memcpy(dst, buf, size);
- return ES_OK;
- }
-
+ /*
+ * This function uses __put_user() independent of whether kernel or user
+ * memory is accessed. This works fine because __put_user() does no
+ * sanity checks of the pointer being accessed. All that it does is
+ * to report when the access failed.
+ *
+ * Also, this function runs in atomic context, so __put_user() is not
+ * allowed to sleep. The page-fault handler detects that it is running
+ * in atomic context and will not try to take mmap_sem and handle the
+ * fault, so additional pagefault_enable()/disable() calls are not
+ * needed.
+ *
+ * The access can't be done via copy_to_user() here because
+ * vc_write_mem() must not use string instructions to access unsafe
+ * memory. The reason is that MOVS is emulated by the #VC handler by
+ * splitting the move up into a read and a write and taking a nested #VC
+ * exception on whatever of them is the MMIO access. Using string
+ * instructions here would cause infinite nesting.
+ */
switch (size) {
case 1:
memcpy(&d1, buf, 1);
- if (put_user(d1, target))
+ if (__put_user(d1, target))
goto fault;
break;
case 2:
memcpy(&d2, buf, 2);
- if (put_user(d2, target))
+ if (__put_user(d2, target))
goto fault;
break;
case 4:
memcpy(&d4, buf, 4);
- if (put_user(d4, target))
+ if (__put_user(d4, target))
goto fault;
break;
case 8:
memcpy(&d8, buf, 8);
- if (put_user(d8, target))
+ if (__put_user(d8, target))
goto fault;
break;
default:
@@ -343,30 +356,43 @@ static enum es_result vc_read_mem(struct
u16 d2;
u8 d1;

- /* If instruction ran in kernel mode and the I/O buffer is in kernel space */
- if (!user_mode(ctxt->regs) && !access_ok(s, size)) {
- memcpy(buf, src, size);
- return ES_OK;
- }
-
+ /*
+ * This function uses __get_user() independent of whether kernel or user
+ * memory is accessed. This works fine because __get_user() does no
+ * sanity checks of the pointer being accessed. All that it does is
+ * to report when the access failed.
+ *
+ * Also, this function runs in atomic context, so __get_user() is not
+ * allowed to sleep. The page-fault handler detects that it is running
+ * in atomic context and will not try to take mmap_sem and handle the
+ * fault, so additional pagefault_enable()/disable() calls are not
+ * needed.
+ *
+ * The access can't be done via copy_from_user() here because
+ * vc_read_mem() must not use string instructions to access unsafe
+ * memory. The reason is that MOVS is emulated by the #VC handler by
+ * splitting the move up into a read and a write and taking a nested #VC
+ * exception on whatever of them is the MMIO access. Using string
+ * instructions here would cause infinite nesting.
+ */
switch (size) {
case 1:
- if (get_user(d1, s))
+ if (__get_user(d1, s))
goto fault;
memcpy(buf, &d1, 1);
break;
case 2:
- if (get_user(d2, s))
+ if (__get_user(d2, s))
goto fault;
memcpy(buf, &d2, 2);
break;
case 4:
- if (get_user(d4, s))
+ if (__get_user(d4, s))
goto fault;
memcpy(buf, &d4, 4);
break;
case 8:
- if (get_user(d8, s))
+ if (__get_user(d8, s))
goto fault;
memcpy(buf, &d8, 8);
break;